The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a local RF and gradient field coil for acquiring NMR images of the human brain.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
The concept of acquiring NMR image data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 32, 64 or 128 views can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantages of echo-planar imaging ("EPI") are well-known, and there has been a long felt need for apparatus and methods which will enable EPI to be practiced in a clinical setting. Other echo-planar pulse sequences are disclosed in U.S. Pat. Nos. 4,678,996; 4,733,188; 4,716,369; 4,355,282; 4,588,948 and 4,752,735.
These fast NMR data acquisition techniques are characterized by the need for rapidly switched gradient fields. As a result, such pulse sequences are not clinically used with commercially available NMR imaging systems because the whole-body gradient coils cannot be switched at the required rate with the available gradient power amplifiers. This is due primarily to the large size and high inductance of such gradient coils. In addition, the high switching rates produce excessive rates of change in the magnetic field in the region of the patient's heart when the patient's head is positioned in the isocenter of the whole-body gradient coils. This FDA limitation on rate of change in magnetic field requires a lowering of the gradient switching rate and a resulting loss in image resolution and/or image quality.
Similarly, the whole-body RF coils provided with commercially available NMR imaging systems are not adequate for EPI imaging. They provide a large field of view to accommodate, for example, the chest and abdominal regions of a human subject, and as a result, their fields couple to large amounts of tissue outside the region of interest being imaged. This lowers the quality factor ("Q") of the RF coil and reduces the signal-to-noise ratio ("SNR") of its signal.
It is well known that the SNR of the RF coil can be significantly improved if it is reduced in size and designed to couple solely with tissues in the region of interest. Such "local" or "surface" coils have been designed for various parts of the human anatomy, such as knees, shoulders, neck, breasts, hands and head. Of particular relevance to the present invention are local head coils which employ the so-called "bird cage" RF coil described by C. E. Hayes et al J. Magn. Reson. 63, 622-628 (1985). See also U.S. Pat. No. 4,694,255. In a birdcage construction, two conductive loops are typically spaced apart along a common longitudinal axis and interconnected by a series of regularly spaced longitudinal conductors.
Similarly, the power required to drive gradient coils at high switching rates can be reduced significantly if they are reduced in size to produce a uniform gradient field only in the region of interest. Thus, local gradient coils have been designed for various parts of the human anatomy such as the wrist, E. C. Wong et al Radiology 181, 393-397 (1991).
In imaging of the human head, the ability to reduce the size of the RF coil and the gradient coils is significantly limited by patient comfort. Although in theory the inner diameter of the RF coil may be reduced to about the diameter of the head of the largest expected human head, such a design risks provoking claustrophobia in the patient. For this reason, in practice, the inner RF birdcage is terminated at approximately the eye level of the patient so that the patient can look down into a larger space of the gradient coil form surrounding the RF coil but at a larger radius. A head coil following this approach is described in U.S. Pat. No. 5,372,137 to Wong et al. issued Dec. 13, 1994 and hereby incorporated by reference. This limited depth head coil reduced the patient's sense of being confined but carries with it a problem of reduced sensitivity to lower portions of the brain, in particular the posterior areas of cerebellum and medulla.